The activated anionic polymerization of lactams is used in numerous different processes for the manufacture of polyamide shaped products. Unproblematic polymerization and the quality of the polyamides prepared by the various processes depend not only on the catalyst but to a large extent also on the nature of the activator used.
Numerous compounds have been used as activators for the anionic polymerization of lactams, i.e. acyl lactams (EP1449865), oxazolines (EP0786482), ethylenebisamides (US2010/0113661), isocyanates and the corresponding masked isocyanate compounds.
Polyisocyanates or isocyanates, particularly diisocyanates, are frequently used as activators. Hexamethylene diisocyanate is particular preferred since it is a highly mobile liquid and as such affords considerable advantage, particularly in continuous process in which accurate delivery by dosing pumps is important. One serious disadvantage of the use of hexamethylene diisocyanate, however is its toxicity which is due to its high vapor pressure and which necessitates special safety measures.
In order to overcome this disadvantage blocked isocyanate compounds have been used. Caprolactam was used as one of such masking agents, forming hexamethylene biscarbamide caprolactam which has been classified as physiologically harmless.
Other liquid activator systems for anionic lactam polymerization are known in which isocyanate compounds are mixed with pyrrolidone compounds, e.g. N-methyl pyrrolidone or N-ethyl pyrrolidone (EP0167907). One serious disadvantage of pyrrolidones, however, is their toxicity. According to EU regulation (EC) No 1272/2008 N-methyl pyrrolidone has been classified as being toxic for reproduction.
EP0697424 A1 discloses compositions comprising organic solvents and polyisocyanates which may be emulgated in water. These compositions are intended for use in the preparation of aqueous dispersions which may in turn be employed as additives for adhesives, impregnating compositions and coating compositions. In order to render the polyisocyanates suitable for forming an emulsion in water, they have at least partially been reacted with compounds providing hydrophilic moieties on the polyisocyanate compounds.
WO 00/42091 discloses an antistatic molded resin article comprising a polyester amide resin prepared by copolymerizing a cyclic amide and specific cyclic esters.
EP0134616 A1 and EP0135233 A1 disclose N-substituted carbamoyl-lactam compounds which are described as suitable promoters or activators for anionic polymerization of lactams. These two prior art documents correspond to the basic concept identified above to use blocked isocyanate compounds.
U.S. Pat. No. 4,757,095 discloses lactones and lactams which may be utilized in the preparation of urethane modified pre-polymers which may be employed for preparing micro cellular foams.
U.S. Pat. No. 5,684,119 discloses directly shapeable clear solutions of polyamide imides which may be employed for the preparation of yarns and fibers.
Additionally, in order to increase the impact strength of the obtained polyamides, impact modifiers, such as polyetheramines have to be added to the anionic polymerization of lactams. However, conventional liquid activator systems coagulate when supplied together with such impact modifiers in the reaction container. To overcome this problem, separate addition means are required, which complicates the polymerization process. Sometimes the impact modifiers are mixed with the polymerization catalyst before the addition to the lactam melt which unavoidably leads to a decrease of the catalyst activity, since polymerization catalysts for anionic lactam polymerization are highly hygroscopic.
An object of this invention is to provide a composition for anionic lactam polymerization which does not contain any pyrrolidone compounds, particularly no N-alkyl pyrrolidone compounds and which is storage-stable and leads to polyamides of excellent quality.